The present invention relates to electric meters and, more particularly, to apparatus and method for providing test operation for electronic demand registers of electric meters.
Conventional electric meters employ an aluminum disk driven as a rotor of a small induction motor by an electric field at a speed which is proportional to the electric power being consumed by a load. Geared dials or cyclometer discs integrate the disk motion to indicate the total energy consumed, conventionally measured in kilowatt hours (one kilowatt hour equals one thousand watts of power consumption for one hour).
In addition to the above measurement of consumption, some electric meters contain demand registers having means for separating the consumption into those parts of consumption occurring during peak and off-peak hours (however defined) and for recording maximum demand during a predetermined period of time in order to adjust billing according to such parameters. In one such meter disclosed in U.S. Pat. No. 3,586,974, a mechanical demand register records the power usage during a predetermined period of time and stores the value for reading. The predetermined period of time may be, for example, the time between meter readings, or a period of time corresponding to the billing period of the utility providing the power. A clockwork mechanism restarts the demand register at regular intervals of, for example, a fraction of an hour, so that, at the end of the predetermined period, the stored value represents the highest value of power usage occurring during any one of the regular intervals in the predetermined period.
Demand registers of the mechanical type, such as disclosed in the above U.S. Patent, have limited flexibility. Once their design is completed for a particular meter physical configuration, the design is not transferrable to a meter having a different physical configuration. Also, the demand-measurement functions cannot be redefined without major mechanical redesign.
When test operation of a mechanical demand meter in the field is desired, there is some question whether such test operation can be performed while abiding by the regulations for demand metering as defined by the commissions having jurisdiction over the utility providing the electric power. Alternatively, if the mechanical demand register is reset in a manner which satisfies the definitions for demand metering, an excessive amount of billing information, with its revenue, may be lost.
Maximum demand metering is conventionally defined as the maximum amount of power consumed in any one contiguous time period during the time interval of interest; that is, the maximum amount of power consumed in any one of the periods of, for example, 15 minutes, 30 minutes or one hour. If test operation is begun in one of such time periods, and the accumulation of actual usage during the time period is paused until the end of the test operation and then resumed, the demand accumulated at the end of the interrupted time period consists of an initial portion before the test interruption and a final portion following the test interruption. The intervention of the test interruption appears to violate the definition of demand metering.
From a practical standpoint, a test interruption may have a significant influence on the reading on the demand register at the end of the interrupted interval. If, for example, power usage during the test interruption would have been very low and power usage in the final portion is, in fact, very high, the high usage in the final portion, added to the usage in the initial portion may rise to a value which remains stored as the maximum demand for the billing period even though the user, except for the test interruption, would not have consumed such a large amount of power in any one contiguous time period.
Conversely, if the final portion is a period of abnormally low usage compared to the usage which would have been recorded during the test interruption, this low usage, when combined with the usage during the initial portion, represents a reduction in billing.
In addition to the above limitations of mechanical demand metering, a useful demand metering technique known as rolling demand is not practically feasible using mechanical demand registers. In rolling demand metering, a demand interval is divided into N contiguous subintervals. The usage during each demand subinterval is summed with the demand recorded during the preceding N-1 subintervals. At the end of each subinterval, the total demand recorded is the demand for N subintervals, i.e. for the entire preceding demand interval. The maximum demand may then be taken as the maximum over any interval sensed at the end of a subinterval. The use of such rolling demand metering avoids distortion in the billing data which could otherwise occur due to short-term extremes in the usage data which would otherwise become lost in the averaging process over an entire demand interval.
Greater flexibility in demand metering may be obtainable using electronic acquisition, integration and processing of power usage. An electronic processor such as, for example, a microprocessor, may be employed to manage the acquisition, storage, processing and display of the usage and demand data. U.S. Pat. Nos. 4,179,654; 4,197,582; 4,229,795; 4,283,772; 4,301,508; 4,361,872 and 4,368,519, among others, illustrate the flexibility that electronic processing brings to the power and energy usage measurement. Each of these electronic measurement devices includes means for producing an electronic signal having a characteristic such as, for example, a frequency or a pulse repetition rate, which is related to the rate of power usage. The electronic processor is substituted for the mechanical demand register of the prior art to keep track of the demand during defined periods of time.
An electronic processor of an electronic demand register conventionally employs volatile random access memory for the high speed and low power consumption characteristics offered by such devices. However, several events can occur during normal and emergency operation of an electric meter containing an electronic demand register which can threaten the integrity of data being recorded for billing purposes in volatile random access memory. If a power outage, by removing power from the processor and the random access memory, were allowed to erase all data stored in random access memory, then the billing data contained in the erased data would be lost. This is, of course, unacceptable. U.S. patent application Ser. No. 599,736, filed on the same date as the present application, discloses means for storing data in non-volatile memory when a power outage occurs and for ignoring or tolerating certain normal deviations of the line power, such as, for example, momentary overvoltage, surges, noise and momentary power outages enduring for a very short time period. This new capability to store billing data and programmed constants in non-volatile memory permits re-thinking the manner in which test operation may be performed and the way in which demand data accumulated before and after a test interruption may be handled so as to provide fairness both to the consumer and to the utility.